What happens if the subject of a 15-6 has his investigation compromised prior to his or her own notification of the 15-6?

Where they talking about the event under investigation or the findings or investigation procedures? In any case, my feeling is that it would be unprofessional, but short of compromising. My guess is that the legal advisor would instruct the IO to not discuss the investigation with outside parties.

The Investigating Officer's Guide only indicates, when considering discussing the investigation:
"In appropriate cases, an investigating officer may direct witnesses not to discuss their statement or testimony with other witnesses or with persons who have no official interest in the proceedings until the investigation is complete. This precaution is recommended to eliminate possible influence on testimony of witnesses still to be heard. Witnesses, however, are not precluded from discussing matters with counsel."

However, it and AR 15-6 are silent on whether the IO should/should not discuss the investigation in an open setting. AR 15-6 does indicate that in the event of "procedural errors or irregularities in an investigation [that] have a material adverse effect on an individual’s substantial rights" should be brought to the appointing officer. In which case, they could direct the IO to correct the issue or appoint a new IO.

One would have to ask if the IO and Commander were discussing how to proceed with a path for the investigation or were they discussing what is being investigated. If they were discussing either in a public setting, I would see that as unprofessional. That being said...if they were ONLY discussing how they will proceed with the investigation but NOT discussing details of the investigation then nothing was compromised. At least, IMHO. If they were discussing specific details as to why the investigation is happening...then yea I could possibly see that as a compromise. If you want a full legal answer, go see JAG.

I think you're confusing a 15-6 investigation for a classified data spill.